INTERACTIONS BETWEEN STALK NUMBER AND STALK WEIGHT AND THE IMPLICATIONS FOR CANE YIELD

YIELD constraints have become an important feature of sugarcane farming systems in Australia, greatly impacting on profitability and productivity. In recent years, there have been two key strategies proposed for overcoming these constraints-manipulating components of the farming system to improve soil health (e.g. crop rotation, controlled traffic and reduced tillage), or increasing plant population by narrowing crop row spacing and increasing planting densities. Both strategies rely on increasing the number of harvested stalks and/or increasing individual stalk size. This paper examines the interaction of these key yield-determining factors against a background of varying environmental conditions. There appear to be clear crop compensatory mechanisms that limit yield responses to increasing stalk densities within a given set of environmental conditions (climate, water availability, soil health). Data clearly show that in the same environmental conditions and with the same levels of resource capture, greater stalk numbers will result in smaller stalk weights. However, once management responses are examined across a range of environmental conditions, crop yield responses will depend on whether crops are capable of producing sufficient extra biomass during stalk filling to meet the demands of additional stalks without a reduction in individual stalk weight. The relative stability of positive yield responses to improved soil health across a range of contrasting environmental conditions suggests this management strategy is generally effective at meeting these conditions, perhaps in part due to a more functional root system that is able to better supply resources to the shoot during the stalk-filling period. The less consistent responses to high-density plantings suggest that, while increases in stalk numbers may be common, the ability to fill those stalks and so capitalise on potential yield increases is less reliable. Higher density plantings may achieve greater canopy closure, but this better developed canopy may result in excessive use of resources (i.e. water in drier environments) or cause undesirable effects like lodging, both of which can reduce growth rates and hence stalk size.